Magnetic fluid seal with precise control of fluid volume at each seal stage

ABSTRACT

A magnetic fluid seal includes a pole ring having an inner diameter, a rotatable shaft having an outer diameter, the rotatable shaft configured to extend along the inner diameter of the pole ring between an atmosphere side and vacuum side, at least one magnet coupled to the pole ring, the at least one magnet configured to emit a magnetic field having a strength and a shape, and grooves formed on either the inner diameter of the pole ring or the outer diameter of the shaft, the grooves capable of containing ferromagnetic fluid. The ferromagnetic fluid contained the grooves varies so as to improve the performance of the magnetic fluid seal.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.12/252,829 filed on Oct. 16, 2008 which claimed priority to U.S.Provisional Patent Application 60/980,977, filed on Oct. 18, 2007, theentirety of all are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to magnetic fluid seal systems and methods formaking the same.

2. Description of the Known Art

During operation of magnetic fluid seals, it has been long observed thatmicrobursts of gas emanate from the magnetic fluid seal into thelow-pressure vacuum as the rotation of the magnetic fluid seal isstarted and stopped. This microburst effect, also known as ‘burping’,results from gas trapped within each seal annulus of the magnetic fluidseal whose pressure exceeds the individual stage gas pressure retentioncapability. As the rotation of the magnetic fluid seal is started andstopped, the dynamic characteristics of the magnetic fluid seal changeslightly, allowing some of the trapped gas to escape into thelow-pressure side of the magnetic fluid seal and into the evacuatedvolume, undesirably raising its overall pressure.

Additionally, in some applications of magnetic fluid seals, it isacceptable to construct the seal by assembling from an atmosphere side.Other applications impose constraints that require the seal to beassembled from a vacuum side. The latter case tends to imposesubstantially greater difficulties in controlling the final distributionof fluid within the seal assembly.

BRIEF SUMMARY OF THE INVENTION

In overcoming the drawbacks of the prior art, a method for making amagnetic fluid seal includes the steps of (1) applying ferromagneticfluid within at least one of a plurality of grooves formed within arotatable shaft or a pole ring, (2) freezing the ferromagnetic fluidplaced within the at least one of the plurality of grooves, and (3)placing the shaft or the pole ring within an opening of a housing of themagnetic fluid seal before the ferromagnetic fluid unfreezes. It shouldbe understood that the opening of the housing of the magnetic fluid sealcan be either an atmosphere side opening or a vacuum side opening, thusallowing the magnetic fluid seal to be assembled from either the vacuumside or the atmosphere side.

By freezing the ferromagnetic fluid, a precise amount of ferromagneticfluid can be held in place during assembly of the magnetic fluid seal.As will be explained in the paragraphs that follow, it has beendiscovered that mircobursting can be minimized or even eliminated usingthis technique. Additionally, it has been discovered that magnetic fluidseals with multiple stages perform better when the amount offerromagnetic fluid placed with the grooves varies as a function of thestrength and shape of a magnetic field. Using the method described inthe paragraphs that follow, one can now precisely vary the amount offerromagnetic fluid placed with the grooves to achieve betterperformance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a view of a portion of an embodiment of a magneticfluid seal embodying the principles of the present invention;

FIG. 2 illustrates a magnified view of a portion of the embodiment of amagnetic fluid seal of FIG. 1; and

FIG. 3 illustrates a cut away view of a portion of a second embodimentof a magnetic fluid seal embodying the principles of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

Referring FIGS. 1 and 2, a first embodiment of a magnetic fluid seal 10is shown. Here, a shaft 12 extends between a vacuum side 14 and anatmospheric side 16. A single piece of ferromagnetic stainless steel,e.g., 17-4 PH alloy or 400-series stainless steel alloy is machined intoa pole ring 18 with an O-ring sealing grooves 20 a and 20 b formed onthe outside diameter and magnetic pole tips 22 on the inside diameter.The pole tips 22 at the inside diameter of the pole ring 18 are machinedas a series of small V-grooves 24 in the inside diameter of the polering 18.

The pole ring 18 is first made with a smooth bore at a carefullycontrolled diameter. Then large slots 26 a and 26 b are machined intothe inside diameter of the pole ring 18. Then the series of V-grooves 24are machined to a depth which leaves a small portion of the originalinside diameter intact between each pair of adjacent V-grooves. FIG. 2shows two slots 26 a and 26 b and a plurality of V-grooves 24 in anarrangement which results in pole tips 22 which are left over from theoriginal inside diameter bore. It is in the gap between these pole tips22 and the shaft 12 that the most intense magnetic field develops, andit is here that the magnetic fluid (represented by “dots” 30 in FIG. 2)is retained by magnetic forces.

The slots 26 a and 26 b are large enough to accept magnets 32 a and 32b, respectively. The slot width is slightly larger than the magnetthickness (e.g., 2.05 mm slot width for 2.00 mm magnet thickness). Thispermits easy insertion of magnets 32 a and 32 b and allows the magnetsto move radially and longitudinally within the slots. As more magnetsare inserted, the mutually repulsive force serves to position eachmagnet 32 a and 32 b equidistant from its neighbors, therebyautomatically providing even spacing throughout the magnet layer.Magnets 32 a and 32 b are added to each slot 26 a and 26 b until theslot cannot accept any more magnets.

Typically the magnets 32 a and 32 b are short cylinders, although theycould also be quadrants, sextants, or octants. Rare earth magnets, suchas SmCo or NdBFe with high energy products (20 to 35 MGO) are preferredto overcome the losses arising from the inherent shunting effectdiscussed below. Magnets 32 a and 32 b are polarized through theirthickness (parallel to the shaft axis). Within each magnet slot 26 a and26 b the polarity is the same. From one slot to the next, the polarityalternates, so that alternate layers of magnets oppose each other. Anynumber of magnet layers can be used, but an even number is preferred(for cancellation of fringe fields). One layer is sufficient for allvacuum applications, although two are normally employed. Forapplications with larger pressure differentials, a greater number oflayers can be used. Note that the outer surface of the pole piece 18 iscontinuous from the atmosphere side to the vacuum side. The continuousouter surface of the pole piece 12 provides a magnetic shunt around eachmagnet. This dissipates some of the magnetic energy which wouldotherwise be available to the magnetic circuits which contain thesealing gaps.

A ferrofluid 22 is provided in the tips 22 and the pole piece 18 isaffixed to a housing 34 (having openings 35 a and 35 b) and the housing34 affixed to a flange (not shown) as described in the parent Helgelandreference U.S. Pat. No. 5,826,885 incorporated herein it its entirety byreference. In turn, the flange can be affixed to a suitable fixturedisposed between the two atmospheres with the shaft 12 extendingtherebetween. It should be understood opening 35 a is directly adjacentto the atmospheric side 16, while opening 35 b is directly adjacent tothe vacuum side 14.

Referring to FIG. 3, another embodiment of a magnetic fluid seal 40 isshown. The magnetic fluid seal 40 includes a housing 42, a pole ring 44and a shaft 46. The pole ring 44 has an inner diameter 48 and an outerdiameter 50. The shaft 46 is rotatable and includes an outer diameter52. The shaft 46 is configured to extend along the inner diameter 48 ofthe pole ring 44 between an atmosphere side 54 and vacuum side. Thevacuum side generally opposes the atmosphere side 54.

The outer diameter 50 of the pole ring 44 is sized and shaped to includeslots 56 a and 56 b. Magnets 57 a and 57 b are placed within slots 56 aand 56 b, respectively. However, it should be understood, as shown inFIG. 1, that the slots may be formed on the inner diameter 48 of thepole ring 44.

The magnets 57 a and 57 b emit a magnetic field having a size and shape.As will be explained later, this magnet field will function to retainferromagnetic fluid in place so as to form a seal between the shaft 16and the pole ring 14.

The shaft 46 includes at least one seal stage 60. As can be seen in FIG.1, the seal stage 60 may be part of a first set 62 of seal stages. Thefirst set 32 includes five seal stages. A second set 64 includes tenseal stages. Although this embodiment shows the first set 62 and secondset 64 of seal stages formed on the outer diameter 52 of the shaft 46,it should be understood that the first set 62 and second set 64 of sealstages may be formed on the inner diameter 48 of the pole ring 44, asshown in FIGS. 1 and 2.

The seal stage 60 generally includes a “V shaped” groove 66 defining apole tip 68. Ferromagnetic fluid is placed within each seal stage 60.When exposed to a magnetic field from the magnets 57 a and 57 b, it isin the gap between the pole tips 68 and pole ring 44 that the mostintense magnetic field develops, and it is here that the ferromagneticfluid is retained by the magnetic field.

Additionally, the inventors have discovered that magnetic fluid sealswith multiple stages perform better when the amount of ferromagneticfluid placed within each stage varies as a function of the strength andshape of a magnetic field. The difficulty in varying the ferromagneticfluid within each stage is that it is difficult to place and holdferromagnetic fluid in the grooves 66 of each stage 60 when the magneticfluid seal 40 is manufactured. Using the method described below, whichcan equally apply to any of the sediments illustrated in FIGS. 1-3, onecan now precisely vary the amount of ferromagnetic fluid placed with thegrooves to achieve better performance.

The method includes the steps of (1) applying ferromagnetic fluid withinat least one of a plurality of grooves 66 formed within the shaft 46 (ora pole ring 18 of FIG. 1), (2) freezing the ferromagnetic fluid placedwithin the plurality of grooves 36, and (3) placing the shaft 46 (or apole ring 18 of FIG. 1) within an opening 67 of a housing 42 of themagnetic fluid seal 40 before the ferromagnetic fluid unfreezes.Generally, the ferromagnetic fluid is applied via a syringe like device.Also, it should be understood that the opening of the housing 42 of themagnetic fluid seal can be either on the atmosphere side 54 or thevacuum side, thus allowing the magnetic fluid seal to be assembled fromeither the atmosphere side 54 or the vacuum side.

The method can further include the steps of applying differing amountsof ferromagnetic fluid within the plurality grooves or even applying theferromagnetic fluid within some of the grooves, while not applying anyferromagnetic fluid in other grooves. Generally, grooves that locatedcloser to the atmosphere side of the magnetic fluid seal are filled withmore ferromagnetic fluid as opposed to the grooves located nearer (oreven adjacent) to the vacuum side of the magnetic fluid seal 10, whichmay contain less or even no ferromagnetic fluid (essentially leaving thestage “dry”). Additionally, the amount of ferromagnetic fluid applied tothe grooves may vary as a function of the strength and shape of themagnetic field produced by the magnets 57 a and 57 b, so as to improveperformance. This variance based on the strength and shape of themagnetic field can be determined by experimentation and empiricalevidence.

It was mentioned in the background section that during operation ofthese types of magnetic fluid seals, it has been long observed thatmicro-bursts of gas emanate from the seal into the low-pressure vacuumas rotation is starts and stops. It has been shown that reducing thepressure of trapped gas within the individual seal stages adjacent tothe vacuum side will minimize or eliminate this microbursting effect. Ithas also been observed that by deliberating creating one or more drystages closest to the vacuum side, then filling one or more sagesimmediately adjacent to those dry stages with ferromagnetic fluid in acontrolled manner described herein, that some of the ferromagnetic fluidwill transfer to the one or more dry stages. During the transferprocess, the gas that is trapped within the effected stage is free to beexpand into the vacuum side of the seal and be evacuated by theprocessing pumps. This leaves very low-pressure gas within the new sealstage and satisfies the condition to prevent microbursting by reducingthe pressure behind the newly formed seal stage. This method is veryimportant for systems that are pumped down to very low pressures thensealed off with little or no active pumping to remove the undesirablegas emanating from the microbursting.

While this invention has been particularly shown and described withreferences to a preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention. Those skilled in the art will recognize or be able toascertain using no more than routine experimentation, many equivalentsto the specific embodiments of the invention described specificallyherein.

1. A method for making a magnetic fluid seal, the method comprising:applying ferromagnetic fluid within at least one of a plurality ofgrooves formed within a rotatable shaft or a pole ring; freezing theferromagnetic fluid placed within the at least one of the plurality ofgrooves, wherein the ferromagnetic fluid changes from a liquid state toa solid state, whereby the ferromagnetic fluid is immobilized; placingthe shaft or the pole ring within an opening of a housing of themagnetic fluid seal before the ferromagnetic fluid changes from thesolid state to the liquid state.
 2. The method of claim 1, wherein theplurality of grooves are formed within the rotatable shaft.
 3. Themethod of claim 1, wherein the plurality of grooves are formed withinthe pole ring.
 4. The method of claim 1, wherein the opening of thehousing of the magnetic fluid seal is directly adjacent to a vacuum sideof the magnetic fluid seal.
 5. The method of claim 1, wherein theopening of the housing of the magnetic fluid seal is directly adjacentto an atmosphere side of the magnetic fluid seal.
 6. The method of claim1, wherein the plurality of grooves extend between an atmosphere side ofthe magnetic fluid seal and a vacuum side of the magnetic fluid seal. 7.The method of claim 6, further comprising the step of applyingferromagnetic fluid within at least one of the grooves located closer tothe atmosphere side of the magnetic fluid seal than the vacuum side ofthe magnetic fluid seal.
 8. The method of claim 1, further comprisingthe step of applying differing amounts of ferromagnetic fluid within theplurality grooves.
 9. The method of claim 8, further comprising the stepof applying differing amounts of ferromagnetic fluid within theplurality grooves based on a strength and a shape of a magnetic field.10. The method of claim 9, wherein the magnetic field is emitted by amagnet coupled to the pole piece.